Conductive paste is a flowable composition composed of a vehicle including a resin binder and a solvent and conductive powder dispersed in the vehicle. It is widely used in the formation of, e.g., electrical circuits, external electrodes of ceramic capacitors, electromagnetic shielding films, and bonding films.
Conductive pastes of that kind are classified into a resin curing type a resin of which cures to cause conductive powder to be compacted to secure electrical connection and a baking type which is baked to cause an organic component to vaporize and conductive powder to be sintered thereby to secure electrical connection.
The former type, resin curing type conductive paste, is a pasty composition usually made up of a conductive powder containing metal powder and an organic binder containing a thermosetting resin, such as an epoxy resin. Upon heat application, the thermosetting resin cures and shrinks together with the conductive powder, whereby the conductive powder particles are compacted in the resin matrix and brought into contact with one another to establish electrical connection. Because the resin curing type conductive paste is processable in a relatively low temperature range of from 100° C. to 200° C. at the highest and less likely to cause great thermal damage, it finds main application in printed wiring boards, thermally sensitive resin substrates, electromagnetic shielding films, bonding films, and the like.
On the other hand, the latter type, baking type conductive paste, is a pasty composition usually made up of a conductive powder (metal powder) and a glass frit dispersed in an organic vehicle. On baking at 500° to 900° C., the organic vehicle vaporizes, and the conductive powder is sintered to establish electrical connection. The glass fit serves to secure the formed conductive film to the substrate, and the organic vehicle acts as an organic liquid medium that makes the metal powder and the glass frit printable.
While the baking type conductive paste may not be used in printed wiring boards or resin materials on account of the high baking temperature, it forms an integral metal layer on sintering to achieve reduced electrical resistance and is therefore used to make, for example, an external electrode of laminated ceramic capacitors.
Since silver has high conductibility, it is widely used as a main constituent material of various conductive materials, such as anisotropic conductive film, conductive paste, and conductive adhesive. For example, a conductive paste prepared by mixing silver powder with a binder and a solvent may be printed on a substrate in a circuit pattern and baked to form an electrical circuit of a printed wiring board or an electronic component.
However, because silver is very expensive, a conductive powder called a silver-coated powder, which is obtained by plating core particles with a noble metal, has been developed and used. For example, Patent Literature 1 below discloses a silver compound-coated copper powder composed of silver-coated core copper particles coated with a silver compound selected from silver oxide, silver carbonate, and an organic acid salt of silver. The silver compound-coated copper powder has an SSA of 0.1 to 10.0 m3/g and a D50 of 0.5 to 10.0 μm and contains 1 wt % to 40 wt % of the silver compound on its surface.
Techniques for coating copper particles with silver include reductive plating and displacement plating.
Reductive plating is a process in which fine particles of silver resulting from reduction with a reducing agent are deposited densely on the surface of copper particles. For example, the method for producing silver-coated copper powder disclosed in Patent Literature 2 below comprises causing metallic copper powder and silver nitrate to react with each other in an aqueous solution having a reducing agent dissolved therein.
Displacement plating is the deposition of metallic silver resulting from reduction of silver ions each gaining an electron from metallic copper on the interface with copper particles, with the metallic copper, on the other hand, being oxidized to copper ions, thereby to replace the surface layer of the copper particles with a silver layer. For example, Patent Literature 3 below discloses a method for producing silver-coated copper powder in which silver is deposited on the surface of copper particles as a result of displacement reaction between silver ions and metallic copper in an organic solvent-containing solution in which silver ions are present.
With regard to silver-coated copper powder per se, Patent Literature 4 below proposes a dendritic conductive powder composed of copper particles having a silver layer on their surface and having a silver content of 3.0 to 30.0 mass % relative to the whole dendritic conductive powder.
Patent Literature 5 below proposes a silver-coated copper powder comprising dendritic silver-coated copper particles having copper particles of which surface is coated with silver and characterized by having a ratio of a BET specific surface area (a specific surface area measured by the BET one-point method) to a sphere-approximated specific surface area (a specific surface area determined using a laser diffraction particle size distribution analyzer) of 6.0 to 15.0.
Patent Literature 6 below discloses a silver-coated copper powder comprising silver-coated copper particles composed of copper particles coated with silver, which is characterized by having a dendritic shape with one main stem and a plurality of branches branched off obliquely from the main stem and grown two-dimensionally or three-dimensionally, the main stem having a thickness a of 0.3 μm to 5.0 μm, and the longest branch having a length b of 0.6 μm to 10.0 μm, as observed under a scanning electron microscope (SEM).